1. Field of the Invention
The present invention relates an electrophoretic display device, particularly, to an electrophoretic display device that can be driven with low power consumption.
2. Description of the Related Art
A reflective display device is low in its power consumption and permits lessening the burden given to the eye of the observer and, thus, is expected to be widely propagated in future. An electrophoretic display device is known to the art as one of the reflective display devices. The electrophoretic display device comprises a pair of electrodes and a suspension loaded in the space between the paired electrodes. The suspension is prepared by dispersing electrophoretic particles having electrical charges in an insulating liquid material. An electric field is applied across the suspension via the paired electrodes. As a result, the electrophoretic particles are allowed to migrate onto the electrode having a polarity opposite to that of the electrical charge of the electrophoretic particles so as to permit the pixel to display a specified color.
In the electrophoretic display device, the electrophoretic particles are colored, and the contrasting color relative to the color of the electrophoretic particles is developed by the medium having dyes dissolved therein. To be more specific, where the electrophoretic particles are attached to the surface of one electrode positioned closer to the observer, the color of the electrophoretic particles is observed. Also, where the electrophoretic particles are attached to the surface of the other electrode positioned remoter from the observer, the color of the electrophoretic particles is shielded by the medium so as to permit the color of the medium to be observed.
The electrophoretic display device is advantageous in that the viewing angle is wide, the contrast is high, and the power consumption is low. However, the electrophoretic display device gives rise to a serious problem that it is essentially impossible to achieve both a high reflectance, i.e., a high brightness, and a high contrast simultaneously because, for example, the die dissolved in the medium may be attached to the particles to color the particles or the colored medium may be entered in the space between the surface of the electrode and the electrophoretic particles attached on the electrode.
A measure against the difficulty noted above is proposed in Japanese Patent Disclosure (Kokai) No. 9-211499 and Japanese Patent Disclosure No. 11-202804. Specifically, proposed in these patent documents is an electrophoretic display device that displays a color by using a transparent medium. In the system disclosed in these patent documents, colored particles are allowed to migrate onto the entire region of the pixel electrode having a size substantially equal to the pixel size so as to display a first color, e.g., black. Also, colored particles are collected on the non-pixel portion or a portion, which has a small area, of the pixel, with the result that the transmitting state of the pixel portion is maintained. As a result, displayed is a second color, e.g., white. In this case, a dye is not dissolved in the medium and, thus, the medium exhibits a high stability. Also, it is possible to achieve a good white display by controlling the scattering characteristics of the reflective electrode.
On the other hand, it is disclosed in U.S. Pat. No. 6,120,588 that fine particles are allowed to migrate by the force other than the electrophoretic force so as to display the color. In the method disclosed in this prior art, an AC electric field is applied across the electrode positioned remoter from the observing surface. As a result, the diffused fine particles formed of a dielectric material are selectively attracted toward the space between a pair of electrodes. It follows that the color of the fine particles left on the observing surface is displayed. In the dielectrophoretic display device utilizing the particular method, pluralities of different kinds of fine particles are selectively attracted downward. Such being the situation, the dielectric characteristics of the colored particles are set to differ from each other, and the plural kinds of the fine colored particles are controlled by the frequency of the AC electric field applied thereto.
However, since the electrophoretic particles are allowed to migrate at a velocity directly proportional to the intensity of the electric field in the electrophoresis, there is no threshold value in the electric field at which migration of the electrophoretic particles is started. Such being the situation, it is said to be substantially difficult for the simple matrix driving or the memory characteristics to be developed. Allowing the electrophoretic particles to be adsorbed on the surface of the electrode can generate the threshold value. However, it is difficult to control the adsorption force so as to give rise to the problem that the threshold value is rendered widely non-uniform or fails to be stabilized over a long period of time.
On the other hand, the display device utilizing the dielectrophoresis is defective in that the response speed is low because of the lack in the restoring force for bringing the fine particles attracted toward the side remoter from the observing surface by the application of an AC electric field back to the side of the observing surface. In addition, since there is no threshold value in the dielectric migrating force under a certain set frequency, it is difficult to cause the fine particles of a specified color to be allowed to migrate selectively with a high accuracy so as to make it difficult to achieve a color display device. It should also be noted that it is necessary to control simultaneously the voltage level and the frequency of the AC voltage applied to a pair of electrodes mounted to each pixel, with the result that a difficulty is also generated in respect of the driving of the display device.